402-r-00-007

United States Office of Radiation and EPA 402-R-00-007 Environmental Protection Indoor Air August 2000 Agency

USER'S GUIDE FORPRESTO-EPA-CPG/POP OPERATIONSYSTEM

Version 4.2

August 2000

Developed by:Cheng-Yeng Hung, Ph.D.

U.S. Environmental Protection AgencyOffice of Radiation and Indoor Air

Washington, DC 20460

USER'S GUIDE FOR

PRESTO-EPA-CPG/POP OPERATION SYSTEM

Version 4.2

August 2000

Developed by:Cheng-Yeng Hung, Ph.D.

U.S. Environmental Protection AgencyOffice of Radiation and Indoor Air

Washington, DC 20460

U.S. Environmental Protection Agency

DISCLAIMER

This user's guide for the PRESTO-EPA-CPG/POP Operation System is the result ofintegrated work sponsored by an agency of the United States Government. Neither the UnitedStates Government nor any agency thereof, nor any of their employees, contractors,subcontractors, or their employees, make any warranty, expressed or implied, nor assume anylegal liability or responsibility for any third party's use of the results of such use of anyinformation, apparatus, product, or process disclosed in this report, nor represent that its use bysuch third party would not infringe privately owned rights.


PREFACE

The mainframe versions of the PRESTO-EPA-POP and PRESTO-EPA- CPG modelswere developed for generating basic data to support EPA's rulemaking on the generallyapplicable environmental standards for the management and disposal of low-level radioactivewaste (LLW).

Since the mainframe versions of the PRESTO-EPA-CPG and PRESTO-EPA-POPmodels were published in December 1987, the Office of Radiation and Indoor Air has receivednumerous requests from potential users urging the Office to convert the models to a form usableon a personal computer. This effort led to the development of a PC version of the PRESTO-EPA-CPG and PRESTO-EPA-POP models. The models were later expanded to includeevaluation of contaminated soil sites as well as LLW sites, were designated as Version 4.0, andwere intended strictly for within-Agency use.

The current model, Version 4.2, combines the previous PRESTO-EPA-CPG andPRESTO-EPA-POP operation systems into one operation system. It updates the user interfaceprogram to facilitate the editing of the input file and viewing of the resultant output file. Theuser interface is a menu-driven program that is supported by the Windows-95 operation systemand performs several data-checking tests. In addition, the model is more flexible for evaluatingsites with various forms of contamination.

The PRESTO-EPA-CPG/POP model predicts the maximum doses to a member of acritical population group and the cumulative numbers of health effects (cancers and geneticeffects) among a general population residing on and downstream of a contaminated soil site, aLLW disposal site, or an agricultural application site. The individuals and populations may beexposed to radioactivity through atmospheric, groundwater, and surface water transportpathways; through drinking water, fish, vegetable, milk, and meat food-chain pathways; andthrough air immersion and direct external exposure pathways. Exposure through inhalation ofdust and radon gas and inadvertent ingestion of soil is also included.


iPresto Users Guide

TABLE OF CONTENTSPage

LIST OF FIGURES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iv

LIST OF TABLES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . v

1 INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-1

1.1 Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-11.2 Changes in Version 2.1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-21.3 Changes in Version 4.2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-3

2 PRESTO-EPA-CPG/POP OPERATION SYSTEM . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-1

2.1 System Installation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-12.2 Start Up . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-12.3 Moving About in the System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-12.4 Scenario Input File Preparation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-2

2.4.1 Input File Parameter Categories . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-32.4.2 Saving an Input File . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-62.4.3 Processing an Input File . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-62.4.4 Description of the Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-7

2.4.4.1 Infiltration Calculations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-82.4.4.2 Initial Calculations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-82.4.4.3 Daughter Nuclide In-Growth Effect Correction Factors . . . . . . 2-82.4.4.4 Annual Summaries . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-82.4.4.5 Maximum Individual Dose Summary or Total Risk Summary . 2-82.4.4.6 Average Individual Dose or Health Effects . . . . . . . . . . . . . . . . 2-8

2.4.5 Graphing the Results of a Processed Input File . . . . . . . . . . . . . . . . . . . 2-92.4.6 PRESTO-EPA-CPG/POP Help . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-92.4.7 Exiting the PRESTO-EPA-CPG/POP Program . . . . . . . . . . . . . . . . . . 2-9

3 THEORETICAL BACKGROUND OF THE PRESTO-EPA-CPG/POP MODEL . . . . 3-1

3.1 General Description of the Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-13.1.1 Description of a Near-Surface Low-Level Radioactive Waste

Disposal Site . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-2 3.1.2 Description of a Contaminated Soil Site . . . . . . . . . . . . . . . . . . . . . . . . 3-2

3.1.3 Description of a Land Farming Site . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-23.2 Mathematical Formulations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-3

3.2.1 Model Assumptions and Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-33.2.2 Transport Pathways Involving Water . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-7

3.2.2.1 Infiltration Through Top Layer . . . . . . . . . . . . . . . . . . . . . . . . . 3-7


ii Presto Users Guide

TABLE OF CONTENTS (Continued)

3.2.2.2 Trench Cap Modification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-113.2.2.3 Rate of Infiltration Through Trench Cap . . . . . . . . . . . . . . . . . 3-143.2.2.4 Rate of Overflow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-143.2.2.5 Radionuclide Leaching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-153.2.2.6 Waste Container Effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-173.2.2.7 Transport Between Contaminated Soil Layers . . . . . . . . . . . . . 3-183.2.2.8 Transport Below Contaminated Zone . . . . . . . . . . . . . . . . . . . 3-19 Vertical Reach . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-19 Horizontal Reach . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-21 Collection Reach . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-223.2.2.9 Radionuclide Breakthrough Time . . . . . . . . . . . . . . . . . . . . . . 3-233.2.2.10 Concentration in the Well Water . . . . . . . . . . . . . . . . . . . . . . 3-233.2.2.11 Rate of Water Consumption . . . . . . . . . . . . . . . . . . . . . . . . . . 3-253.2.2.12 Surface Stream Contamination . . . . . . . . . . . . . . . . . . . . . . . 3-26

3.2.3 Atmospheric Transport Sources and Pathways . . . . . . . . . . . . . . . . . . 3-283.2.3.1 Internal Model Capability and Formulation . . . . . . . . . . . . . . . 3-293.2.3.2 Source Term Characterization . . . . . . . . . . . . . . . . . . . . . . . . . 3-323.2.3.3 Transport Formulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-333.2.3.4 Effects of a Stable Air Layer on Transport . . . . . . . . . . . . . . . 3-343.2.3.5 Effects of Plume Depletion . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-35

3.2.4 Food Chain Calculations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-373.2.5 External Radiation Risk to Residents . . . . . . . . . . . . . . . . . . . . . . . . . 3-46

3.2.5.1 Area Factor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-463.2.5.2 Radiation Risk from Outdoor Ground Surface . . . . . . . . . . . 3-473.2.5.3 Basement Risk to Residents . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-49 Side-Wall Layer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-49 Floor Layer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-50

3.2.6 Radiation Risk from Radon Inhalation . . . . . . . . . . . . . . . . . . . . . . . . 3-523.2.6.1 Diffusion Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-533.2.6.2 Advection Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-543.2.6.3 Indoor Radon Concentration . . . . . . . . . . . . . . . . . . . . . . . . . . 3-583.2.6.4 Outdoor Radon Concentration . . . . . . . . . . . . . . . . . . . . . . . . . 3-593.2.6.5 Dose from Radon Inhalation . . . . . . . . . . . . . . . . . . . . . . . . . . 3-60

3.2.7 DOSTAB Calculations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-613.2.7.1 Radiological Doses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-61

3.2.8 Health Effects Estimates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-633.2.9 Daughter Nuclide In-Growth Effect Correction . . . . . . . . . . . . . . . . . . 3-64

3.2.9.1 Decay Chains . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-633.2.9.2 Dose Equivalent Calculation . . . . . . . . . . . . . . . . . . . . . . . . . . 3-653.2.9.3 Cumulative Health Effects Calculation . . . . . . . . . . . . . . . . . . 3-66

3.3 Health Effects Induced in the Regional Basin Population . . . . . . . . . . . . . . . . 3-683.3.1 Calculations of Regional Basin Health Effects . . . . . . . . . . . . . . . . . . 3-69


iiiPresto Users Guide

TABLE OF CONTENTS (Continued)

3.3.2 Conversion Factors for Regional Basin Health Effects . . . . . . . . . . . . 3-713.3.2.1 Terrestrial Pathway . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-713.3.2.2 Fish Pathway . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-733.3.2.3 Basin Health Effects Conversion Factor . . . . . . . . . . . . . . . . . 3-74

REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . R-1

APPENDIX A THEORETICAL BACKGROUND OF THE INFILTRATION SUBMODEL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A-1

APPENDIX B THEORETICAL BACKGROUND OF THE GROUNDWATER TRANSPORT SUBMODEL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B-1

APPENDIX C THEORETICAL BACKGROUND OF DAUGHTER NUCLIDE IN-GROWTH EFFECTS CORRECTION FACTOR . . . . . . . . . . . . . . C-1

APPENDIX D DEVELOPMENT OF EQUIVALENT UPWARD DIFFUSIVITY AND CONDUCTIVITY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . D-1

APPENDIX E BENCHMARK STUDY FOR THE DEGREE OF SATURATION FORMULA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . E-1

APPENDIX F THEORETICAL BACKGROUND OF EXTERNAL EXPOSURE CALCULATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . F-1

APPENDIX G THEORETICAL BACKGROUND OF THE AREA FACTOR SUBMODEL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . G-1

APPENDIX H THEORETICAL BACKGROUND OF THE WELL WATER CONCENTRATION CALCULATION . . . . . . . . . . . . . . . . . . . . . . . H-1

APPENDIX I INPUT PARAMETER LIST AND DESCRIPTION . . . . . . . . . . . . . . . I-1 APPENDIX J-1 REFERENCE INPUT FILE FORMAT FOR PRESTO-CPG . . . . . . . . J-1

APPENDIX J-2 REFERENCE INPUT FILE FORMAT FOR PRESTO-POP . . . . . . . J-13

APPENDIX K SAMPLE OUTPUT FILE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . K-1


iv Presto Users Guide

LIST OF FIGURESPage

2-1 File Screen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-2

2-2 Input Parameter Screen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-3

2-3 Site Input Parameter Screen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-4

2-4 Source Input Parameter Screen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-5

2-5 Save As Screen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-6

2-6 Process Screen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-7

3-1 Environmental Transport Pathways . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-4

3-2 Water Transport Exposure Pathways . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-8

3-3 Trench Cap Failure Function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-13

3-4 Airborne Exposure Pathways in PRESTO Codes . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-31

3-5 Diffusive and Advective Radon Models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-53

3-6 Advective Flow Region Around Basement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-57

3-7 Radon Diffusion Through Soil Layers and Cover Defects . . . . . . . . . . . . . . . . . . . . . 3-59


vPresto Users Guide

LIST OF TABLESPage

1-1 Function of PRESTO-EPA Family of Codes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-2

3-1 Exposure Pathways . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-6

3-2 Soil Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-11

3-3 Leaching Options (LEAOPT) Specified . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-15

3-4 Unit of Exposure and Dose Rate Factors Used in DOSTAB . . . . . . . . . . . . . . . . . . . . 3-62


1-1Presto Users Guide

1. INTRODUCTION

1.1 BACKGROUND

Under the Atomic Energy Act, as amended, the U.S. Environmental Protection Agency(EPA) has the authority to develop generally applicable standards for the disposal of low-levelradioactive waste (LLW). Technical support for the standards includes an estimation of thehealth impacts from the disposal of LLW in a wide variety of facility types located in diversehydrogeological settings.

As an aid in developing the standards, a family of PRESTO (Prediction of RadiationEffects from Shallow Trench Operation) codes, entitled PRESTO-EPA-POP,PRESTO-EPA-DEEP, PRESTO-EPA-CPG, PRESTO-EPA-BRC, and PATHRAE-EPA, hasbeen developed under EPA direction (EPA87a through EPA87g.) The PRESTO-EPA-POP codewas the first code developed and served as the basis for the other codes in the family. EPA usesthe PRESTO-EPA code family to compare the potential health impacts (cumulative populationhealth effects and maximum annual dose to a critical population group) to the general public andcritical population group for a broad number of LLW disposal alternatives. Table 1-1 provides abrief description of the function of each member of the code family. The application of thesecodes has been described in detail elsewhere (Hu83a, Gal84, Ro84, Mey81, Mey84).

The PRESTO-EPA-CPG and PRESTO-EPA-POP codes were designed to estimate themaximum annual committed effective dose (CED) and the annual mortality and cancer incidencerisks to a critical population group (PRESTO-EPA-CPG) and the number of fatal cancer deathsand serious genetic effects to the general population (PRESTO-EPA-POP) resulting from thedisposal of LLW. The codes were later modified to include the modeling of contaminatedsurface soil sites and were designated Version 4.0 of the PRESTO-EPA-CPG/POP model.

The model calculates the health impacts exposure to the individuals and populationsthrough atmospheric, groundwater, and surface water transport pathways; drinking water, fish,vegetable, milk, and meat food-chain pathways; and air immersion and direct external exposurepathways. Exposure through inhalation of dust and radon gas and inadvertent ingestion of soilare also included.

The model described in this report is an extension of the original PRESTO-EPA-CPG andPRESTO-EPA-POP models that makes them more flexible for evaluating sites with variousforms of contamination. The most significant modification to the operation system is thecombination of the previous PRESTO-EPA-CPG and PRESTO-EPA-POP operation systems intoone operation system.


1-2 Presto Users Guide

Table 1-1. Function of PRESTO-EPA Family of Codes

Code Purpose

PRESTO-EPA-POP

PRESTO-EPA-DEEP

PRESTO-EPA-CPG

PRESTO-EPA-BRC

PATHRAE-EPA

PRESTO-EPA-CPG/POP

Estimates cumulative population health effects to local andregional basin populations from land disposal of LLW by shallowmethods; long-term analyses are modeled (generally 10,000years).

Estimates cumulative population health effects to local andregional basin populations from land disposal of LLW by deepmethods.

Estimates maximum annual whole-body dose to a criticalpopulation group from land disposal of LLW by shallow or deepmethods; dose in maximum year is determined.

Estimates cumulative population health effects to local andregional basin populations from less restrictive disposal of belowregulatory concern (BRC) wastes by sanitary landfill andincineration methods.

Estimates annual whole-body doses to a critical population groupfrom less restrictive disposal of BRC wastes by sanitary landfilland incineration methods.

Estimates maximum doses, risks, and cumulative populationhealth effects from LLW disposal sites, contaminated soil sites,land application sites, or emergency response sites.

The user interface is a menu-driven program that is supported by the Windows-95operation system. The program is user-friendly and is designed to reduce human errors and tofacilitate the preparation of the input file and the display of the output file.

1.2 CHANGES IN VERSION 2.1

Version 2.1 is the first version ever published since the model was converted to operateon a personal computer. An interface program, written in a DOS operation system, was alsoincorporated into the model to form an operation system. Four improvements were made to thisversion: (1) the addition of the daughter nuclide in-growth effects (DNIE) into the riskassessment, (2) the update of the dose and risk conversion factors to the 1994 level, (3) theaddition of the annual mortality and risk incidence calculation, and (4) the adoption of theInternational System (SI) units.



The dose coefficients are extracted from the RADRISK data file (Du80) and theweighting factors are consistent with the definitions used in International Commission onRadiological Protection Publications 26 (ICRP77) and 30 (ICRP79). The effective doseequivalent is the weighted sum of the 50-year committed dose equivalent to the organs or tissues.

The cancer risk coefficients are calculated from radiation-risk models that are based on1980 U.S. vital statistics. The genetic-risk coefficients for serious disorders to all subsequentgenerations are calculated from the product of the average absorbed dose to the ovaries and testesup to age 30 per unit intake before that age. Risk coefficients of 2.60x10-2 and 6.9x10-2 Gy-1 forlow-LET and high-LET radiation, respectively, are used for the calculation of risk conversionfactors (EPA89).

The Version 2.1 Operation System modifies the PRESTO-EPA-CPG model byintegrating the daughter nuclide in-growth effects into the Version 2.0 model. The DNIE arecalculated based on a crude assumption that the sorption characteristics of the parent anddaughter nuclides are identical throughout the processes of leaching and groundwater transport. The DNIE are adjusted annually by using the correction factors derived from Bateman Equations(Ev55).

The adjustment for DNIE is performed only for those parent nuclides designated and builtinto the model. To simplify the modeling, the adjustment is carried up to four-member decaychains. The transport of daughter nuclides is not calculated in the model.

1.3 CHANGES IN VERSION 4.2

Five further improvements were made to this version of the model: (1) the expansion ofthe model applicability from the field of radioactive waste disposal to the field of soil cleanup,agricultural land application, land reclamation, and accidental spill, (2) the combination ofPRESTO-EPA-CPG and PRESTO-EPA-POP into one operation system, (3) the addition of aradionuclide-specific database, (4) the improvement of user friendliness by converting theinterface program to a Windows system, and (5) the addition of data collection guidelines forthose input data not commonly known.

This version of the PRESTO-EPA-CPG/POP model not only evaluates exposures fromnear-surface low-level radioactive waste, as did the original PRESTO-EPA-CPG and PRESTO-EPA-POP models, but it has been expanded to include repeated agricultural applicationoperations, long-term impacts from accidental spills, and contaminated soil sites. However, nochange was made to the modeling of exposures from LLW disposal operations.

The radionuclide-specific data are collected for 825 radionuclides. The system willautomatically pull out necessary data from the built-in databases based on the nuclides that theuser selects. However, the maximum number of radionuclides that the user can select remains at40.



In response to user requests, the model now includes input data guidelines for the inputparameters, upward equivalent hydraulic conductivity and diffusivity used for the infiltrationcalculation, and the parameters, residual and saturated water contents used for the unsaturatedzone radionuclide transport calculation. The guidelines are now included in the documentationfor clay, loam, and sand.



2. PRESTO-EPA-CPG/POP OPERATION SYSTEM

The PRESTO-EPA-CPG/POP operation system is a menu-directed, user-friendly system. The system is designed to help the user of the PRESTO-EPA-CPG/POP model to easily preparethe input characterizing a scenario of interest files and to graphically view results. This chaptercontains step-by-step instructions on installing the system, and creating, processing, and viewingthe results of a scenario input file.

2.1 SYSTEM INSTALLATION

The PRESTO-EPA-CPG/POP operation system is designed to operate in the MicrosoftWindows 95 environment. The computer should be equipped with at least 16 megabytes ofRAM and have a minimum of 10 megabytes of available disk storage.

To install the PRESTO-EPA-CPG/POP operation system, either from a set of 3.5 inchdiskettes or from a downloaded folder, proceed as follows:

1. A. 3.5 inch diskettes: Insert the PRESTO-EPA-CPG/POP diskette 1 into drive A.

B. Downloaded folder: Stores all files in the folder.

2. Click the Start button and select Run.

3. Type a:\setup.exe (where a: is your floppy drive) or c:\download\setup (if optionB is used), and click OK.

4. Follow the setup instructions on the screen.

2.2 START UP

To begin a guided tour of the PRESTO-EPA-CPG/POP operation system, access thePrograms option under the Start button and then click on the PRESTO-EPA-CPG/POP globeicon.

2.3 MOVING ABOUT IN THE SYSTEM

You can select an option in the PRESTO-EPA-CPG/POP operation system by either (1)clicking on the selection with the mouse or (2) moving to the selection with the keyboard usingthe Tab, Shift-Tab, or arrow keys. The menu options may also be selected by pressing andreleasing the Alt key, followed by the appropriate highlighted letter.



2.4 SCENARIO INPUT FILE PREPARATION

The structure of the PRESTO-EPA-CPG/POP operation system consists of severaloptions, each of which performs a designated function. The options shown under the File menubar allow you to create a new input file, open an existing input file, and process one or moreinput files. A maximum of three input files may be opened at one time.

Figure 2-1. File Screen

As shown in Figure 2-1 above, you can open an existing file or create an input file byactivating the File menu option and then selecting New or Open. Alternatively, you can pressand release the Alt key and then press F to activate the File menu option. To create a new input,select New. A window will appear prompting you to select the type of input file to create. Thetwo available options/scenarios include either a CPG (Critical Population Group) input file or aPOP (Population) input file. The CPG scenario estimates radiological doses to a criticalpopulation group and the POP scenario estimates the cumulative health effects to the generalpopulation. The type of file is indicated by the extension of the file name: cpg for criticalpopulation group and pop for general population. The file is named Input File 1 until it issaved with a different name.



To access an existing file, select Open and specify the type of input file to open. Thescreen will now show a window with additional options that allow you to enter or modify inputparameters.

2.4.1 Input File Parameter Categories

After you have opened an existing or new input file, the window will display six tabs thatcontain parameter descriptions and fields (Title, Control, Site, Basement, Source, andResults). These options allow you to modify or view parameters in a logical and organizedmanner. In addition to the options shown in the main menu, the Site and Source options containa second level of options that will be discussed in more detail below. Appendix I contains alisting of the input parameters sorted by screen. The list also includes the units of each parameterand ranges of acceptable values.

Figure 2-2. Input Parameter Screen

As shown in Figure 2-2 above, from the main menu, select the Title option if it is notvisible. The screen will show a window with several fields and field descriptions for eachparameter. You can select input fields by either clicking on the field with the mouse or pressingthe Tab key until the desired field is active. You can also see its specified unit by pointing andholding the mouse over the field. You can now input parameters by simply typing in the data and



then pressing the Tab key to select the next field. The other options are similar to the Titleoption in functionality.

Figure 2-3. Site Input Parameter Screen

Figure 2-3 above demonstrates that the Site tab contains a second level of tabs that arecategorized by input parameters relating to plant, animal, and human uptake factors, cover andwaste layers, vertical zone and aquifer, and the atmosphere. The definition of each inputparameter is briefly presented on the screen. However, Site Environment, which is in the pull-down list located under the Atmosphere tab, requires additional explanation. This parameterindicates region-specific meteorological data to be selected for the analyses. Region-specificdata are given for humid south, humid north, arid south, and arid north environments. You canalso select the User Defined option when you want to create a user-defined meteorological datafile (INFIL.INP). This file must be modified externally in a text editor.

The Source tab contains options relating to nuclide-specific data such as uptake factors,transport parameters, waste inventory, and dose and health effect conversion factors (see Figure2-4 on the following page). Dose and health effect conversion factors are further grouped bypathway (i.e., ingestion, inhalation, immersion, and surface). The Source tab allows you to enter



Figure 2-4. Source Input Parameter Screen

nuclide-specific parameters into the input file. After you have selected this option, a newwindow appears with a list of 825 nuclides in alphabetical order. You can select a maximum of40 nuclides to be included in the input file by double-clicking on the nuclide or by pressing theSpace bar once the cell is active. The total number of nuclides selected is automatically updatedand shown at the top of the window. Use the same method to deselect nuclides from the list.

When a nuclide is added to the input file, the program automatically searches the doseand health effects conversion factor database for a default value (EPA88, EPA93, EPA99). If avalue is found in the database, it is added to the input file. After you have selected the desired setof nuclides to include in the input file, you may re-enter new values for the dose and riskconversion factors. In addition, select the Uptake, Transport, and Inventory options to enterthe appropriate nuclide-specific parameters. You also have the option of entering theradionuclide inventory in units of Bq or Ci by selecting the appropriate option at the top of theInventory window.

2.4.2 Saving an Input File

To save the current input file, select the Save As option in the File menu bar, as shown inFigure 2-5 on the following page, and enter the desired filename. You can also change the pathby choosing a different directory and drive. Once you have entered the filename, select the Save



option to save the input file or the Cancel option to return to the main menu without savingchanges to the current file. When changes to an input file have not been saved, the system willwarn you when you try to close the file. The same warning message appears if you try to exit theprogram without saving changes to the current file. The physical structures of the input files aredocumented in Appendix J-1 for PRESTO-CPG and Appendix J-2 for PRESTO-POP. They areprovided for user reference only; you are strongly encouraged to modify these files only fromwithin the operation system.

Figure 2-5. Save As Screen

2.4.3 Processing an Input File

To process an input file, choose the Process option in the File menu bar (see Figure 2-6on the following page). Locate the directory that contains the input files and then select at leastone input file to process. A window will appear showing the file being processed and the numberof input files that have yet to be processed.

After processing the input file, view the output by selecting the Results option. Thesystem will prevent you from accessing this option until the input file has been processed. Theoutput is organized into the following six categories:



Figure 2-6. Process Screen

1. Infiltration,2. Initial Calculations,3. Daughter Nuclide Effect Correction Factors,4. Annual Summaries,5. Maximum Individual Dose Summary or Total Risk Summary, and6. Average Individual Dose or Health Effects.

The default units representing radionuclide activity and radiological doses are expressedas FSv and Bq. The units for activity and dose, however, can be changed to mrem and Ci byselecting the desired option shown on any one of the tabs.

2.4.4 Description of the Results

The results of PRESTO-EPA-CPG/POP are designed to be self-explanatory and containdescriptive comments, definitions, and intermediate and final tabulations. It is assumed that theresults may be analyzed by users unfamiliar with PRESTO-EPA-CPG/POP structure. Asillustrated by the sample input and output in Appendix K, the PRESTO-EPA-CPG/POP resultsare organized into the following sections.



2.4.4.1 Infiltration Calculations

The first section consists of the results for the infiltration calculations. The modelcalculates and outputs several data items. The most important of these are annual infiltration andannual precipitation. Annual evaporation, runoff, and infiltration are also calculated and output.

2.4.4.2 Initial Calculations

A set of tables under the heading Initial Calculations summarizes the radionuclide dataused for the transport calculations. These tables include radionuclide distribution coefficientsand nuclide inventories for both contaminated soil layers.

2.4.4.3 Daughter Nuclide In-Growth Effect Correction Factors

This section lists the daughter nuclide effect correction factors for each radionuclide. Thederivation of these correction factors is presented in Appendix C.

2.4.4.4 Annual Summaries

Input control parameters determine the years for which intermediate results are printed. For these years, a number of hydrological and transport variables are output. Included are thestatus of the top layer (percentage intact), water depth in the contaminated zone, water loss byoverflow and drainage from the contaminated zone, and radionuclide inventories. Radionuclideconcentrations and flux values are also given for key pathways and regions of interest. Intermediate whole body doses to the critical population group or fatal cancer and genetic risks tothe general population are other important results given in this section of the PRESTO-EPA-CPG/POP output.

2.4.4.5 Maximum Individual Dose Summary or Total Risk Summary

For the critical population group scenario, this section contains the data and resultsdescribed in Section 2.4.4.4 for the year in which the maximum critical population dose occurs. This allows for specific identification of contributing pathways and radionuclides.

For the population scenario, this section contains the total risk summary for the entiresimulation. These tables present a summary of the cumulative cancer incidence, cancermortality, and genetic effects, the cumulative radioactivity pumped out of the well, and thecumulative release of radionuclides to the downstream basin for each radionuclide.

2.4.4.6 Average Individual Dose or Health Effects

For the critical population group scenario, the last section is similar to the resultsdescribed in Section 2.4.4.5 except the results apply to the average exposed individual. For thegeneral population scenario, this section provides the total number of fatal health effects andgenetic health effects resulting from the contaminated soil site.



2.4.5 Graphing the Results of a Processed Input File

After processing either a CPG or a POP input file, you may graph the results by selectingthe Graph option in the menu bar. For CPG results, the graphing options include: (1) individualdose, (2) mortality risk, (3) incidence risk, (4) well water concentration, and (5) total annual dose. For POP results, the graphing options include: (1) fatal cancer risks and (2) genetic risks. Afteryouve selected a graph, a new window will appear allowing you to select a combination ofnuclides and/or pathways to include in the graph. To view the graph by nuclide or pathway,select the appropriate tab.

You can modify the appearance of the graph by selecting the Edit option in the File menubar. The chart style, text, and axis scale and type (i.e., linear or logarithmic) are a few of theformatting options that can be modified. Additionally, you may copy the graph and paste it intoother applications by selecting the Copy option in the Chart menu bar.

The Print option in the Chart menu bar allows you to print the graph. The layout andformatting of the printed graph can be adjusted by selecting the Layout option on the Printdialogue box.

2.4.6 PRESTO-EPA-CPG/POP Help

At any time, you may access the Help option to view online help. PRESTO-EPA-CPG/POP Help contains instructions on using the system and detailed information on all of theinput parameters. The help system also contains a table of contents, an index, and a search tab tohelp you locate the desired topic.

Additionally, parameter-specific help is provided whenever you place the mouse cursorover a parameter. The information displayed on the screen includes the parameter name, units ofthe parameter, and, if applicable, ranges of acceptable input values.

2.4.7 Exiting the PRESTO-EPA-CPG/POP Program

To exit the PRESTO-EPA-CPG/POP program, select the Exit option in the File menubar. A warning message will appear on the screen if you have not saved changes to an openedinput file. Select Yes to save and exit the program. Select No to exit without saving changes. Toremain in the system, select the Cancel option.



3. THEORETICAL BACKGROUND OF THEPRESTO-EPA-CPG/POP MODEL

3.1 GENERAL DESCRIPTION OF THE MODEL

The model has been designed to calculate the maximum annual committed effective dose(CED) to a critical population group (CPG) and cancer incidence, fatal cancer effect, and seriousgenetic effect in the general population resulting from a near-surface disposal of low-levelradioactive waste (LLW) site, from cleanup of a radiologically contaminated site, and frombeneficial application/disposal of naturally occurring radioactive material (NORM) waste. Themodel simulates the transport of radionuclides from a site to the environmental receptors andhuman exposure through food chain pathways. The health effects for the general population arecalculated from the radionuclide uptake rate and the health effect conversion factors.

The model assumes that the radionuclides may contaminate the near-ground surface intwo soil layers upper and lower layers having different soil characteristics and contaminationlevels. Based on the nature of the exposure to the contaminants, three types of human receptorsare considered: (1) onsite residents, residing on the contaminated site; (2) offsite residents,residing near the site; and (3) the general population, residing in the downstream basin, whichextends all the way to the estuary.

The PRESTO-EPA-CPG model calculates the maximum individual dose to the onsite andthe offsite residents depending on which is the critical population group, and the PRESTO-EPA-POP model calculates the cumulative number of fatal cancer effects and serious genetic effects tothe onsite residents, offsite residents, and general population.

The onsite residents are assumed to be exposed to the contaminant through ingestion of(1) the drinking water pumped out from the groundwater and surface water contaminated,respectively, from subsurface leaching and overland flow; (2) foods (crops and cattle) grown onthe farmland contaminated from initial contamination, subsequent chronic application (NORMwaste disposal, contaminated irrigation water), and redeposit of radionuclides being transportedfrom upstream through atmospheric process; and (3) fish caught from nearby contaminatedsurface waters, and through soil, inhalation of radon gas outdoors and indoors, direct exposureoutdoors and in the basement, and immersion in the air that is resuspended from local soil or istransported upstream.

The offsite residents are assumed to be subject to the same exposure as the onsiteresidents except that there is no initial contamination of soil in the resident area and the exposurefrom radon gas is direct exposure from living in the basement. In addition, the offsite residentsare assumed to be exposed to the radionuclides being transported from the contaminated sitethrough atmospheric transport, which contaminates the foods grown in the area.

The general population is assumed to be exposed to all pathways that the offsite residentsexperience, except the groundwater and atmospheric transport pathways.



3.1.1 Description of a Near-Surface Low-Level Radioactive Waste Disposal Site

The life cycle of a LLW disposal site begins with site selection. Following site selectionand regulatory approval, trenches are dug on the site. Waste materials, which may be acombination of trash, absorbing waste activate metal, and solidified wastes, are containerized andplaced into each trench. Once a section of the trench is filled, the trench is backfilled to eliminatevoids to decrease the potential for subsidence and cracking of the trench cap. Followingbackfilling, the trench is covered with a cap of soil, one to several meters thick, and moundedabove grade to facilitate runoff and decrease infiltration. The cover soil may be clean orcontaminated with spilled radionuclide.

The site is assumed to have a definite period of institutional control following thecompletion of the disposal operation. During the institutional control period, the cap is assumedto be maintained in its intact condition. However, the site is assumed to be a restricted land-usearea and no residents are allowed to reside onsite.

The model can assess the health impacts to offsite residents and the general populationdownstream for various combinations of waste form and disposal operation alternatives.

3.1.2 Description of a Contaminated Soil Site

A contaminated soil site is a site with residual radionuclides resulting from the cleanup ofan accidental spillage or the decommission of a nuclear facility. Unlike a radioactive wastedisposal site, the contamination generally does not extend more than a few meters below theground surface. However, the radionuclide release and transport processes at a contaminated soilsite are similar to those at a radioactive waste disposal site. The contaminated soil site may bemodeled as a single layer of contamination (with or without a clean soil cover layer) or as twodistinct layers of contamination.

The model can assess the health impacts to onsite residents, offsite residents, and thegeneral population.

3.1.3 Description of a Land Farming Site

A farmland application of NORM waste contaminated with a diminished level ofradioactivity is normally used for the purpose of disposal and/or to benefit the land. Waste isapplied to the surface of the selected site and then mixed, using a traditional plow, to a nominaldepth. This operation can be either a one-time application or repeated annually for a givennumber of years. No cover or cap is present in this scenario. Additionally, no waste treatment,such as solidification or containerization, is modeled.

The model can evaluate long-term effects to onsite residents, offsite residents, and thedownstream general population.



3.2 MATHEMATICAL FORMULATIONS

3.2.1 Model Assumptions and Structure

The PRESTO-EPA-CPG/POP operation system, which includes the PRESTO-EPA-CPGand PRESTO-EPA-POP models and their interface programs, is designed to accommodate awide range of hydrogeologic and climatic conditions. It is also designed to handle the leachingof radionuclides and their subsequent groundwater transport through unsaturated and saturatedhydrogeologic conditions, while taking into account nuclide retardation due to geochemicalprocesses. The systems features account for the dynamic leaching process, the farming scenario,which simulates farming over the waste with root uptake of radionuclides from the waste matrix,and the reduction in the source inventory due to radionuclide decay during the simulation.

In general, hydrologic transport is the principal pathway by which the human receptorsmay become exposed to the radioactivity contaminating a disposal/contaminated soil site. Figure 3-1 shows a schematic of the pathways through which the water may transportradionuclides from a site to human receptors. The major source of water, which is the primarydriving force for leaching the radionuclides out of the contaminated matrix, is from precipitation. The precipitated water at a site will either infiltrate into the soil layers, run off the site byoverland flow, or evaporate into the atmosphere.

The transport of radionuclides from a site may occur by the infiltrated water or by theoverland flow. A dynamic model, which calculates the evaporation loss and water transportbased on their dynamic equations, is used to calculate the rate of infiltration (Appendix A). Theinfiltrated water entering the contaminated zone leaches out radionuclides from the contaminatedzone. This contaminated water may either overflow from the top of the site or percolatedownward to the subsoil and ultimately enter an aquifer.

Radionuclides that finally reach the aquifer will generally be transported at velocities lessthan or equal to the flow velocity of the water in the aquifer. This "retardation" is due to theinteraction of radionuclides with solid media in the aquifer, known as the sorption effect. Whenthe radionuclides being transported in the aquifer reach a well, they will be consumed by onsiteand/or offsite residents through drinking, irrigation, and cattle feed pathways. Residualradionuclides in the aquifer are assumed to be transported farther downstream and imposeadditional health impacts on downstream general populations.



Figure 3-1. Environmental Transport Pathways



The contaminated water in the site will accumulate if the rate of infiltration exceeds therate of exfiltration out the bottom of the contaminated zone. When the volume of wateraccumulated in the waste exceeds the total void space, contaminated water overflows onto theground surface. The radionuclides in the contaminated water will then mix with the overlandflow and be further transported into nearby streams.

This contaminated water will potentially be consumed by the local residents and by thepopulation downstream via drinking, irrigation, cattle feed, and fishing pathways.

The model incorporates the effect of daughter nuclide in-growth in its final results bymultiplying the parent-nuclide-caused health effects with its daughter nuclide in-growth effectcorrection factor, which is defined in Section 3.2.9. Up to the fourth member of the decay chainis included in this adjustment.

The complex physical and chemical interactions between the radionuclides and the solidgeologic media have been grouped into a single factor, the distribution coefficient (Kd). Different Kd values can be used for the top contaminated soil layer, the bottom contaminated soillayer, the subsoil, and the aquifer.

The model assumes that the leaching process takes place in both the upper and lower soillayers. The top layer takes the infiltrated rainwater as its driving force to leach out radionuclidesfrom the soil, while the bottom layer receives the leachate from the top soil layer and uses it asthe driving force to leach the radionuclide in the bottom soil layer. Negative leaching, orsorption, may occur when the concentration of the radionuclides in the leachate exceeds theequilibrium concentration of radionuclides in the bottom soil layer.

The subsurface transport path of radionuclides is assumed to be vertical from the waste tothe aquifer and then horizontal through the aquifer. The flow in the vertical flow regime iscalculated either as saturated or unsaturated flow, depending on the relationship among the rateof exfiltration, the degree of saturation, and the properties of the geologic media. The transportof radionuclides in the aquifer is calculated by employing Hung's "optimum groundwatertransport model" (Hu81), in which Hung's correction factor is used to compensate for the effectsof longitudinal dispersion (given in Appendix B).

Three types of submodels are used in the PRESTO-EPA-CPG/POP model: unit response,bookkeeping, and scheduled event. The unit response submodels calculate the annual responseof a given process. For example, the submodel INFIL calculates the annual infiltration throughan intact top soil layer. This annual infiltration is then apportioned among the transportprocesses by the bookkeeping submodels. Other unit response models calculate the annualaverage atmospheric dispersion coefficient and erosion from the top layer.

Bookkeeping submodels keep track of the results of unit response submodels anduser-supplied control options. For example, the TRENCH submodel calculates the level ofstanding water in the source-contaminated layers and the volume of water leaving thecontaminated zone.



Annual concentrations of each radionuclide in environmental receptors, such as wellwater or the atmosphere, are used to calculate radionuclide concentrations in foodstuffs. Foodstuff concentrations and average ingestion and breathing rates are utilized to calculate theannual average radionuclide intake per individual in the onsite and local populations. Theseintake data are then used to estimate doses and health effects.

The atmospheric transport submodel assumes a constant dust loading and mixing heightfor estimating onsite concentrations. For offsite atmospheric transport, the system assumes thatthe entire local population resides within the same 22.5-degree sector. User-specified parametersgive the fraction of year that the plume blows in that sector. The transport of the radionuclidesfrom the source area to the local population is calculated by employing the Gaussian plumediffusion model. Therefore, each member of the local population will inhale the same quantity ofeach radionuclide.

Each person in a specific local community is assumed to consume the same quantities andvarieties of food, all grown in the same fields, and to obtain his or her drinking and cropirrigation water from the same source. However, the user may specify the distribution of thesources of drinking and irrigation water supplies between well and stream. Table 3-1 lists theenvironmental pathways by which members of the onsite, offsite, and general populations areexposed to radioactivity.

Table 3-1. Exposure Pathways

Onsite Residents Offsite Residents General Population

Drinking water Drinking water Drinking water

Crop irrigation Crop irrigation Crop irrigation

Cattle feed Cattle feed Cattle feed

Fish ingestion Fish ingestion Fish ingestion

Soil ingestion Soil ingestion Soil ingestion

Dust inhalation Dust inhalation

Outdoor direct exposure(ground surface and air)

Outdoor direct exposure(ground surface and air)

Basement gamma exposure

Radon inhalation(indoor/outdoor)

Radon inhalation(outdoor)

The mathematical formulation adopted by the model in calculating radionuclideconcentrations in the pertinent environmental receptors is described in the following twosections.



3.2.2 Transport Pathways Involving Water

Figure 3-2 shows the surface water and groundwater transport pathways and theirassociated exposure pathways (see Section 3.2.1).

3.2.2.1 Infiltration Through Top Layer

The basic model for simulating the annual infiltration through the contaminated soil isbased on the cap failure function in an interim version of the model. It assumes that a portion ofthe top layer material will degrade and allow the precipitated water to flow through the lowercontaminated layer. The fraction of the top layer that fails is assumed to vary with time. Due tothe distinct nature of the infiltration mechanism between the intact portion and the failed portionof the top layer, the annual infiltration through the top layer is divided into two components.

On the intact portions of the layer, the normal infiltration rate is calculated by the methoddeveloped by Hung (Hu83b), which is described in Appendix A. For the failed portion of the toplayer, the infiltration equals the rainfall. Therefore, the volume of water entering the trenchannually is calculated by:

Wt = At[fc " Ws + (1 - fc)Wa] (3-1)

where

Wt = volume of water entering trench in current year (m3),

At = area of trench (m2),

fc = fraction of top layer that has failed (unitless),

Wa = annual infiltration through intact cap or top layer (m),

Ws = annual infiltration through bottom layer where cap has failed (m)= (Pa + Ir)fb

Pa = annual precipitation (m),

Ir = annual irrigation rate (m), and

fb = fraction of infiltration through bottom layer (unitless).



Figure 3-2. Water Transport Exposure Pathways



The value of Wt is added to the standing water from the earlier year to calculate themaximum depth of standing water in the soil for the current year.

The component of annual infiltration through the intact portion of the top layer, Wa, isestimated by employing the infiltration model developed by Hung (Hu83b, Appendix A). Themodel simulates the rate of infiltration by solving system equations that describe the dynamics ofoverland flow, subsurface flow, and atmospheric dispersion systems. The basic equationsemployed in the model are:

Q0 = {(Sin2) H5/3}/n (3-2)

dH/dt = P - Eo - qo - Qo/L (3-3)

(3-4)E

E P H/ t E

P H/ t P H/ t

. P H/ t

p P

0 0

0 0

=

+ >

+ + >

+ =

when

when

when

(3-5)q

K P E H t K

P E H t K P E H t

P E H to

S S

S=

+ >

+ > + >

+ =

when

when

when

0

0 0

0

0

0 0

/

/ /

. /

(3-6)qK Z Z

Z ZiS g

g

=

>

=

when

whenmax

max.0

dZg/dt = (qi - qo + qt)/Wg (3-7)

qL = -DeWp/Zp + Ke(3-8)

qL < Ep - Eo

(3-9)q E EZ

W Wv p op

p g

= +

+

( ).

. ( )1

05

0 66

1

dZp/dt = -(qp + qt)/Wp (3-10)

(3-11)qq Z

ZtP

P

=

>

=

0 0

0 0

when

when.



and qp = -Max(*qL*,*qv*) (3-12)

where

Qo = rate of overland flow per unit width of trench cover (m3/m-hr),

H = average depth of overland flow over the entire trench cover (m),

L = length of slope or half of trench width (m),

n = Manning's coefficient of roughness,

2 = average inclination of the trench cover (m/m),

P = rate of precipitation (m/hr),

Eo = rate of evaporation from the overland flow (m/hr),

qo = rate of percolation from the overland flow system (m/hr),

Ep = evaporation potential (m/hr),

qi = flux of moisture infiltrating into the trench (m/hr),

qL = flux of pellicular water transported in the liquid phase (m/hr),

Ks = saturated hydraulic conductivity of the soil (m/hr),

Zg = deficit of gravity water (m),

Zmax = maximum deficit of gravity water, equivalent to the thickness of the trenchcover (m),

Wg = component of wetness for the gravity water; under a fully saturatedcondition, it is numerically identical to the porosity for the gravity water(unitless),

Wp = component of wetness for the pellicular water; under a fully saturatedcondition, it is numerically identical to the porosity for pellicular water(unitless),

Zp = deficit of the pellicular water (m),

De = hydraulic diffusivity at equivalent wetness (m2/hr),



Ke = hydraulic conductivity at equivalent wetness (m/hr),

qv = flux of moisture being transported in the vapor phase (m/hr),

qt = flux of moisture being transformed from gravity water to pellicular water(m/hr), and

qp = flux of pellicular water (m/hr).

The amount of annual infiltration through the top soil layer or trench cap is thencalculated by integrating the hourly infiltration over the entire year.

The values of hydraulic conductivity and diffusivity to be used in the calculation of theflux for the pellicular water are theoretically functions of water content (see Equation 15,Appendix A). In order to simplify the calculation, the water content independent terms upwardequivalent hydraulic conductivity and upward equivalent hydraulic diffusivity are introducedfor the submodel. They are calculated from the soil characteristics obtained from soil tests.

For the purpose of modeling input data collection, three typical soils with appropriate soilcharacteristics are selected and their equivalent values are calculated. Details of thederivation/calculation are attached in Appendix D. The results of the calculation are presented inTable 3-2 below. These values are intended for use as guidelines in preparing the inputparameters for equivalent conductivity and diffusivity.

Table 3-2. Soil Characteristics

Variables Sand Loam Clay

upward diffusivity, m2/hr 7.2E-4 3.6E-4 1.4E-4

upward hydraulic conductivity, m/hr 3.6E-6 1.4E-6 1.1E-6

3.2.2.2 Trench Cap Modification

The trench cap may fail by erosion or mechanical disturbance. In the case of erosion, theannual thickness of material removed from the top layer by sheet erosion is calculated using anadaptation of the universal soil loss equation (USLE) (Wi65).

The annual amount of erosion is subtracted from the layer thickness for the current yearof simulation. If the remaining thickness is less than 1 cm, the cap is considered to becompletely failed and fC is set to 1.0. The USLE may be written as:

Il = fr " fk " fl " fs " fc " fp " fd (3-13)



where

Il = yearly sediment loss from surface erosion (tons/ha),

fr = rainfall factor (fr unit or 100 m-tons-cm/ha),

fk = soil erodability factor (tons/ha/fr-unit),

fl = slope-length factor (unitless),

fs = slope-steepness factor (unitless),

fc = cover factor (unitless),

fp = erosion control practice factor (unitless), and

fd = sediment delivery factor (unitless).

The parameterization scheme of McElroy et al. (McE76) was used to specify site-specificvalues of the factors in Equation (3-13). The rainfall factor, fr, expresses the erosion potentialcaused by average annual rainfall in the locality. The soil erodability factor, fk, is also tabulatedby McElroy et al. as a function of five soil characteristics: percent silt plus very fine sand;percent sand greater than 0.1 mm; organic matter content; soil structure; and permeability. Thefactors fl and fs, for slope-length and steepness, account for the fact that soil loss is affected byboth length and degree of slope. The PRESTO-EPA-POP code usage of USLE combines bothfactors into a single factor that may be evaluated using charts in McElroy et al.

The cover factor, fc, represents the ratio of the amount of soil eroded from land that istreated under a specified condition to that eroded from clean-tilled fallow ground under the sameslope and rainfall conditions. The erosion control practice factor, fp, allows for reduction in theerosion potential due to the effect of practices that alter drainage patterns and lower runoff rateand intensity. The sediment delivery ratio, fd, is defined by McElroy et al. as the fraction of thegross erosion that is delivered to a stream. Units of Il are converted to (m/yr) within the code. See Appendix I for the description of input units.

The second method of top layer failure is mechanical disturbance due to human intrusionor some other means, such as channel erosion, which might completely destroy portions of thetop layer. This phenomenon can be termed a partial failure, but in reality it is a total failure ofsome part of the top layer. The code user may specify some rate of failure as shown inFigure 3-3.



f

, t < NYR

(PCT PCT )(t-NYR )

(NYR -NYR )

PCT , t > NYR

PCT ,NYR t NYRc =

+

0 1

2 1 1

2 1

2 2

1 1 2

Figure 3-3. Trench Cap Failure Function

By specifying appropriate values for the time in Figure 3-3, the user may selectivelysimulate the failure of the cap or top layer from a portion of the site area. Mathematically, thisfunction is represented by:

(3-14)

Even though PCT2 might be less than 1.0 in year NYR2, the cap may ultimately fail completelyby virtue of erosion. As fC changes, the amount of water added to the trench annually alsochanges.



3.2.2.3 Rate of Infiltration Through Trench Cap

The amount of water infiltrating through and leaving the bottom layer annually iscalculated by Darcys law but cannot exceed the available volume of water remaining in the soillayer, which is expressed mathematically in:

VB = Min(DW " AT , IT"AT) (3-15)

in which DW is calculated by:

DW = VW/(AT " WT) (3-16)

In above equations:

VB = volume of water leaving contaminated zone annually (m3/yr),

DW = depth of water in contaminated zone during current year (m),

IT = conductivity of material below the contaminated zone (m/yr),

AT = trench area (m2),

VW = volume of water in contaminated zone (m3), and

WT = porosity of soil (unitless).

3.2.2.4 Rate of Overflow

Water will overflow the site if the maximum depth of standing water is greater than thecontaminated zone depth. If this is the case, the overflow is calculated by:

VO = (DW - DT)AT " WT (3-17)

where

VO = volume of water overflowing site in a year (m3),

DW = depth of water in contaminated zone (m),

DT = contaminated zone depth (m),

AT = site area (m2), and

WT = porosity of soil material (unitless).



3.2.2.5 Radionuclide Leaching

Water in the contaminated zone may be contaminated by contact with the radioactivity. To calculate the concentration of radionuclides in the water exfiltrating out of the site, two modeltypes are used, a dynamic model based on the chemical exchange and an empirical model basedon the annual release fraction.

The user must choose one of the three options shown in Table 3-3 to calculate theconcentration of radionuclides in the exfiltrating water.

Table 3-3. Leaching Options (LEAOPT) Specified

Option Leach Calculation Method

1

2

3

Chemical exchange without solubility limit

Chemical exchange with solubility limit

Annual release fraction

Leaching options 1 and 2 utilize a dynamic model that estimates the radionuclideconcentration in the water based on chemical exchange. When this model is selected, annualrelease rates are required for solidified wastes and activated metals. Contaminants released fromthese waste forms are then treated to chemical exchange, as is the absorbing waste.

The model is developed based on a multi-phase leaching concept (Hu86b), whichsimulates a leaching system under a field environment. The model assumes that the flow ofinfiltration is concentrated in preference paths and, thereby, forms a finger flow system. Thisflow system leads to the transport of radionuclides in two phases, the convective phase and thediffusive phase. These phases of transport are assumed to take place in the convective zone andthe diffusive zone, respectively. The radionuclides in the diffusive zone must be transported tothe convective zone before they can be transported downward through the convective process.

Due to the complexity in the modeling of the multi-phase leaching concept, a simplifiedand yet conservative model is used. The simplified model assumes an idealized steady, uniformleaching model to calculate the radionuclide concentration in the trench water based on thechemical exchange process. A correction factor is then added to account for the leaching processunder field conditions derived from the multi-phase leaching concept (Hu86b). The finalformula is expressed by:



IT " FACCTW =

AT(DTWT + DTKd2DW) (3-18)

(Chemical exchange option)

and

FAC = Min [TINFL/PERMT, 1] (3-19)

where

FAC = a correction factor to account for the multi-phase leaching phenomenonexperienced in field conditions,

TINFL = effective annual infiltration rate (m/yr),

PERMT = soil hydraulic conductivity (m/yr),

CTW = concentration of radionuclides in contaminated water (Bq/cm3),

IT = amount of activity in layer (Bq),

AT = site area (m2),

WT = porosity within soil layer (unitless),

DT = layer depth (m),

Kd2 = distribution coefficient within waste for radionuclides (ml/g), and

DW = density of waste material (g/cm3).

Leaching option 2 uses a solubility factor to estimate the maximum concentrations ofradionuclides in the leachate. The solubility option may be used when the radionuclide solubilityis low or information concerning Kd values is not available. The radionuclide concentration isestimated by:

(3-20)C MinS N N

M,

I FAC

D A W A D KTWc v T

T T T T T d2 W

=

+

(Solubility Option) where



CQ f Q

WTWU L P

=

+

S = elemental solubility (g/ml),

M = mass of radionuclide (g/mole),

Nc = ratio (Bq/mole), and

Nv = ratio (ml/m3).

Leaching option 3 allows the user to input an average annual fractional release of the totalradionuclide inventory. This fraction is applied to each radionuclide and does not consider eitherKd or solubility. Leaching option 3 is normally used for a solidified waste form. The modelcalculates the primary release of radionuclides from the waste form by using a user-specifiedconstant-fractional leach rate. To accommodate the hydrodynamic effects, the releasedradionuclides are then adsorbed by the waste form according to Equation (3-18) to calculate theactual rate of release out of the trench. This calculation accounts for the adsorption effects insideand outside of the waste form.

If the constant fraction release model is chosen, the radionuclide concentrations in thewater leaving the waste are given by:

(3-21)

where

CTW = nuclide concentration in water leaving the waste (Bq/m3),

Q = total unleached inventory (Bq),

fL = annual leach fraction (1/yr),

QP = leached activity in waste left from previous year (Bq), and

W = volume of water in contaminated zone (m3).

3.2.2.6 Waste Container Effects

Waste containers can inhibit nuclide leaching until they lose their integrity. The length oftime before containers lose their integrity container life depends on their design, structuralstrength, and material. In PRESTO-EPA-CPG, the net radionuclide release is calculated bymultiplying the radionuclide concentration in the trench water by the fractional container fracturefactor (CFF), which is time-dependent. The fraction CFF is set to zero while all of the containersare intact. Once the containers start to fracture, CFF is assumed to increase linearly to amaximum value of 1, which represents failure of all of the containers.



3.2.2.7 Transport Between Contaminated Soil Layers

The leaching of the contaminated soil layers is modeled in three layers: active, top, andbottom layers. The active layer is subdivided from the top layer and remains the same thicknessthroughout the entire period of analysis, while the thickness of the remaining top layer decreaseswith the progress of the sheet erosion. The three-layer leaching model is based on the above twoequations with the added capability that radionuclides may transfer from one contaminated layerto the other. For groundwater pathway calculation, the three-layer leaching model isimplemented by first applying the leaching equations to the active layer to get a leachateconcentration. The amount of radioactivity transferred from the active layer to the top layer orfrom the top layer to the bottom layer is the product of the leachate concentration in the layer andthe volume of deep infiltrating water. That is:

PA1 = CWA @ Vb(3-22)

P12 = CW1 @ Vb

where

PA1 = radioactivity transferred from active layer to top layer (Bq),

P12 = radioactivity transferred from top layer to bottom layer (Bq),

CWA = leachate concentration in active layer (Bq/m3),

CW1 = leachate concentration in top layer (Bq/m3), and

Vb = volume of water leaving bottom of waste (m3).

In some cases, site conditions do not allow the downward movement of water through thewaste. In these cases, the waste becomes saturated and water overflows from the site. This leadsto upward movement of radioactivity from the bottom layer to the top layer and from the toplayer to the active layer. The amount of radioactivity transferred upward to the top and activelayers is the product of the leachate concentration in the layer and the volume of water thatoverflows from the site. Mathematically it can be represented by the following equation:

P1A = CW1 @ V0(3-23)

P21 = CW2 @ V0

where

P1A = radioactivity transferred from top layer to active layer (Bq),

P21 = radioactivity transferred from bottom layer to top layer (Bq),



SSAT RESAT RESAT)ATINFL

PERMV= +

(

.

10 25

Cw1 = leachate concentration in top layer (Bq/m3),

Cw2 = leachate concentration in bottom layer (Bq/m3), and

Vo = volume of water overflowing the site (m3).

These equations are implemented in the LEACH subroutine, which is called by the mainprogram for every radionuclide for each year of the simulation. As in the original PRESTOcodes, the leached radionuclides either travel downward to the groundwater pathway or flowacross the ground surface to the surface water pathway.

3.2.2.8 Transport Below Contaminated Zone

Once radionuclides have been leached out of the waste in the contaminated zone, they aretransported vertically downward to the aquifer and then horizontally through the aquifer to awell. The velocity of radionuclide transport is retarded, relative to the movement of water, byvertical and horizontal retardation factors, RV and RH, as explained below.

Because of the distinct nature of radionuclide transport in various reaches, the modelsubdivides the transport field into three reaches: vertical reach, collection reach, and horizontalreach. The solute transport analyses for each reach are conducted as detailed in the followingsubsections.

Vertical Reach

The groundwater flow in the vertical reach is assumed to be saturated or partiallysaturated. The degree of saturation is used to calculate the water velocity, Vv, and the verticalretardation factor, RV. The degree of saturation, SSAT, is either read in as an input parameter orcalculated from the equation:

(3-24)

where

RESAT = fraction of residual moisture content relative to saturated moisture content(unitless),

ATINFL = average exfiltration rate (m/yr), and

PERMV = saturated hydraulic conductivity in the vertical reach (m/yr).

Equation (3-24) is an empirical formula for predicting the fraction of saturation in a soilunder dynamic infiltrating conditions as developed and applied by Clap and McWhorter (Cla78,



McW79). The exponent, 0.25, should theoretically be a function of soil type, but a conservativefixed value was selected based on the experimental data for simplicity. In order to justify thereliability of this equation, a benchmark study is conducted to compare it with another commonlyused empirical formula. The results of the comparison fit quite well with each other. Details arepresented in Appendix E.

In Equation (3-24), the residual moisture content is the moisture content remaining in thesoil after long-term drainage through gravity, and it is equivalent to the component of moisturecontent for the hygroscopic water. RESAT is the ratio of this water content and the saturatedwater content and is a user input parameter. As a guideline for selecting this value, typical valuesfor sand, loam, and clay soils are calculated based on the soil test data obtained in the laboratory(Hil76). Details of this calculation is presented in Appendix E. The results are 0.25, 0.42, and0.77 respectively for sand, loam, and clay soils.

The parameter ATINFL is the average trench exfiltration rate. When there is no overflowof trench water, the rate is calculated by the expression:

ATINFL = [PCT2 " (PPN+XIRR)+(2 - PCT2) " XINFL] " 0.5 (3-25)

where

PCT2 = maximum fraction of trench cap failure (unitless),

PPN = annual precipitation rate (m/yr),

XIRR = annual irrigation rate (m/yr), and

XINFL = infiltration rate through the intact top layer (m/yr) (calculated by the INFILsubroutine).

Vertical water velocity, Vv (m/yr), and the vertical retardation factor, Rv (unitless), arecalculated as follows:

Vv = ATINFL/(PORV SSAT) (3-26)

Rv = 1 + (BDENS XKD3)/(PORV SSAT) (3-27)

where

BDENS = host formation bulk density (g/cm3),

XKD3 = distribution coefficients for the host formation (ml/g), and



PORV = subsurface porosity (unitless).

Horizontal Reach

The transport analysis for the horizontal reach calculates the radionuclide transport in theaquifer without lateral or vertical supply of radionuclide flux. The transport analysis employsHung's groundwater transport model (Hu81, Hu86a, Appendix B). The basic equations for themodel, as adopted from Hung, are:

Q = 0Qo(t-RL/V+tL) Exp(-8dRL/V+tL) (3-28)

and

4 0.5(RP/B23)1/2 Exp[-Nd2-(P2/4R)(R/2-1)

2]d2

0 = m0 ))))))))))))))))))))))))))))))))))))))))))))) Exp(-RNd)

(3-29)

Exp[P/2 - (P/2)(1 + 4RL8d/PV)1/2]

= )))))))))))))))))))))))))))))))) Exp(-RL8d/V)

where

0 = Hung's correction factor, to compensate for the dispersion effect,

R = retardation factor,

P = Peclet number, VHDH/d,

2 = dimensionless time, JV/L,

Nd = decay number, 8dL/V,

L = flow length, DV or DH (m),

V = water flow velocity, VV or VH (m/yr),

t = time of simulation (yr),

d = dispersion coefficient (m2/yr),



Q t B x q tRx

VU t

Rx

VExp

Rx

Vdxd

L

( ) ( ) ( ) ( ) ( )=

0

8d = radiological decay constant (yr-1),

J = dummy time variable (yr),

Q = rate of radionuclide transport at the point of interest, which is at well pointin this case (Ci/yr),

Qo = rate of radionuclide released at the upstream reach, which is at thedownstream edge of a disposal site (Ci/yr), and

tL = time a lechate control system is in use (yr).

In the above equation, the horizontal retardation factor, RH, is calculated by

RH = 1 + (BDENS XKD4)/PORA in which

XKD4 = distribution coefficient of the aquifer (ml/g), and

PORA = aquifer porosity (unitless).

Collection Reach

This analysis calculates the rate of radionuclide transport in the aquifer while receivingthe radionuclide flux from the vertical reach. The primary purpose of the analysis is to calculatethe rate of radionuclide transport at the downstream edge of the site boundary.

The basic equation used to calculate the rate of transport at the site boundary is expressedas:

(3-30)

in which

Q = rate of radionuclide transport at the downstream edge of the disposal site(Ci/yr),

B = width of the disposal site measured in the direction perpendicular to thegroundwater flow (m),

L = length of the disposal site in the direction of groundwater flow (m),



0(x) = Hung's correction factor for the flow reach from the downstream edge ofthe disposal site to the point of integration,

q = radionuclide flux entering the aquifer at the point of integration (Ci/yr/m2),and

U = unit step function.

To simplify the calculation, Hung's correction factor, 0(x), is assumed to be equal to 1.0in the actual model analysis. This approximation is acceptable because the length of theintegration reach should not exceed the length of the disposal site, which is relatively small, andthe 0(x) value is almost always 1.0 under normal application. Furthermore, the model assumesthe segment of integration, dx or )x, to be one-hundredth of the length of the disposal site inconducting the numerical integration.

3.2.2.9 Radionuclide Breakthrough Time

The breakthrough time, which is the time required for a radionuclide to travel from thebottom of the trench to the well, is the sum of the vertical and horizontal transit times. From apractical viewpoint, the breakthrough time is approximated in the model by assuming that theradionuclide leaching is from a point source and that the dispersion effect on the radionuclidetransport can be neglected. The vertical and horizontal transit time, tV (yr) and tH (yr), arecalculated according to:

DVRV DHRH tv = )))))) , tH = )))))) (3-31)

VV VH

where

DV = distance from trench to aquifer (m),

DH = length of aquifer flow from trench to well (m),

VV = vertical water velocity (m/yr), and

VH = water velocity in aquifer (m/yr),

and retardation factors RV and RH are as previously defined.

3.2.2.10 Concentration in the Well Water

Since the well point receptor for the calculation of maximum annual committed effectivedose is, in general, fairly close to the edge of the disposal site, the concentration of radionuclidesin the well water may vary considerably with the depth of the well screen installed. In addition,



the well may pump in dilution water from the stream layer below the screen level. The thicknessof this layer is a complicated function of the aquifer characteristics and the rate of pumping.

Theoretical concepts used in the development of the basic equation for the calculation ofwell water concentration is detailed by Hung (Hu99) and is attached as Appendix H.

The PRESTO-EPA model assumes that the well screen is installed near the bottom of theaquifer, which is the most reasonable assumption based on current well drilling practice and State well water regulation in the United States. Furthermore, the model also assumes that thewell will withdraw water uniformly from the layer of stream between the water surface to thebottom of the well screen.

The depth of the screen is the user-assigned depth and the potential dilution water thatmay be withdrawn from the layer below the screen is considered negligible. The overallassumptions tend to overestimate the concentration of radionuclide and is considered to be aconservative approach.

To calculate the radionuclide concentration at the well point, the rate of groundwater flowat the well point is calculated first. By considering the lateral dispersion of the flow, the totalrate of flow available for dilution is calculated by:

WA = VAPADA[B + 2 tan("/2)DH] (3-32)

where

WA = the rate of contaminated water flow in the plume at the well point (m3/yr),

VA = groundwater velocity (m/yr),

PA = porosity of aquifer material (unitless),

DA = depth of well penetrating into the aquifer (m),

" = constant angle of spread of the contaminant plume in the aquifer (radian),

B = site width (m), and

DH = distance from the center of site to the well (m).

The angle """ is the dispersion angle of a contaminated plume in the water in an aquifer. This dispersion angle may be empirically determined (e.g., by field dispersion tests wherein theangle of dispersion is determined from measurements of chemical, conductivity, or radioactivitytracers in water from a series of boreholes downstream across the plume), or it may be estimated. The use of a dispersion angle is consistent with published characterizations of the horizontallyprojected profile of a chemical contamination front as it moves through an aquifer (Sy81).



The radionuclide concentration in the well water, CW (Ci/m3), is calculated by:

CW = Q/WA (3-33)

where Q is the rate of radionuclide transport at the well point.

3.2.2.11 Rate of Water Consumption

The total water demand, VU, including drinking water, cattle feed, and crop irrigation, iscalculated by:

VU = [3.9E-7 WIfILI + UWLH + 1.5E4 LA]Np (3-34)

where

Vu = annual well water demand in liters (l/person"yr),

3.9E7 = 4492 m2 irrigated per person X 8760 hr/yr,

WI = irrigation rate per unit area (l/m2"hr),

fI = fraction of year when irrigating (unitless),

UW = individual annual water consumption (l/person"yr)

LH = fraction of drinking water obtained from well water,

1.5E4 = water fed to cattle consumed by humans (l/person"yr),

LA = fraction of cattle feed water obtained from well water,

NP = size of the population (persons), and

LI = fraction of irrigation water obtained from well water.

If the calculated total water demand, Vu, exceeds the flow rate of the contaminated plume,WA, the concentration of radionuclides in the pumped-out water is recalculated using the actualvolume of pumping to correct for the dilution effect from the noncontaminated groundwater. Units of VU are converted to cubic meters within the code.

The calculated concentrations of radionuclides in well water are averaged over the lengthof the simulation and used by the food chain and human exposure parts of the code for thedrinking water and cattle feed pathways.



3.2.2.12 Surface Stream Contamination

As previously mentioned, water will overflow the site onto the soil surface when themaximum depth of standing water is greater than the contaminated zone depth. If this occurs,radionuclides will be added to the surface inventory. The surface soil will then have acomponent adsorbed by the soil with concentration CSS (Ci/kg) and a component of contaminatedwater in the surface soil of CSW (Ci/m

3). The material adsorbed by the soil remains in the soil andbecomes a source term for resuspension and atmospheric transport (discussed in Section 3.2.3). The contaminated water in the surface soil is available to enter nearby surface water bodies viaoverland flow, or to percolate down to the aquifer.

Radionuclides dissolved in the soil water may be transported either to the stream byoverland flow or to the deeper soil layers by percolation. The amount of each radionuclide addedto the stream is represented by the product of CSW, the radionuclide concentration in the surfacesoil water, and the annual volume of runoff from the contaminated soil surface, WSTREM. Thevalue of CSW for each radionuclide is calculated by:

1000 ISCSW = )))))))))))))) (3-35)

Kd1MS + MW2/DW

where

CSW = radionuclide concentration in surface soil water (Bq/m3),

IS = amount of radionuclide on surface (Bq),

Kd1 = distribution coefficient for surface soil region (ml/g),

MS = mass of soil in contaminated region (kg),

MW2 = mass of water in contaminated soil region (kg),

DW = density of water (g/cm3), and

1000 = conversion factors used for Kd(1 ml/g = 1 m3/1000 kg) and for DW (1 g/cm

3

= 1000 kg/m3).

Equation (3-35) is used to compute the concentration of radionuclides in the surface soilinterstitial water.

The radionuclide concentration in the contaminated surface soil region, CSS, is calculatedusing:

CSS = CSWKd1/1000 (3-36)



The contaminated region of surface soil is defined by the area of the site and has a user-specified depth or thickness. These parameters allow the calculation of soil mass (MS) and watermass (MW) in the contaminated soil region by:

MS = 1000 DSSwSLSD, MW = 1000 WSSWSLSD (3-37)

where

DS = soil bulk density (g/cm3),

WS = soil porosity (unitless),

SL = length of the site (m),

SW = width of the site (m),

SD = depth of the active layer (m), and

1000 = conversion factor for the mass of soil and water.

Water falling on the contaminated soil region may either evaporate, run off, or infiltrate. A certain fraction of the total precipitation, fr, will enter the stream annually.

The amount of water that enters the stream from runoff of the contaminated region isgiven by:

WS = frPSWSL (3-38)

where P is the annual precipitation.

The amount of water that enters deep soil layers and, eventually, the aquifer is given by:

WD = WaSWSL (3-39)

where Wa is the yearly infiltration rate for the farmland.

The annual amount of radionuclides moving from the contaminated surface soil region tothe stream, RS, is then the product of WS and the radionuclide concentration in the surface soilwater, CSW (Equation (3-35)). The amount of each radionuclide annually entering the deeper soillayers from the contaminated surface soil region is the product of WD and CSW. The conc

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